They will spread!
The native bluebell is under threat of being out competed by the cultivated spanish bluebell, which will happily cross breed and pruduce hybrids which then overgrow the native parent clumps.
I have just being involved with a scheme to take native bluebell seed to Queensland Australia, to grow in the Tamborine Mountain Botanic Gardens. I delivered the seed by hand in Sept. 2004, and they are being raised there now.
I recommend taking a pack of seed through customs, (I did the paperwork first), but we were the only people on the flight to have anything to declare, and were the first to get through!
On my visit to Queensland I did notice some spanish bluebells in gardens around the area, so the plan to have an isolated gene pool of native English bluebells in Australia looks like it will fail before it gets started. The Australians were very happy to try the experiment though, and you never know with plants!
Brian